In addition to the other great answers, there is a 1-handed method for tying the bowline around your body, which could come in handy if your other hand is busy hanging on. A good thing to know!
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saratogahikerJul 17 '14 at 15:15

4 Answers
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The bowline knot is very safe if loaded correctly. This is the usual, safe way to load it:

The chair foot is the body (sorry for not offering naked models), the part of the rope leading away from the picture will take the load. In this use case the knot should hold perfectly. On the other hand, you might get the idea to use the bowline knot to create a sling, say to rappel down from a tree:

This is wrong and if you rappel down from that carabiner you might get yourself killed, because the knot can slip when loaded “sideways”, pulling the main loop apart. There are other similar wrong use cases, all depending on the knot being able to hold when pulled sideways – which the bowline knot isn’t.

This was discovered during an investigation of a fatal fall in Germany in the 1960s. The case went to the court and after that the Germans have done a series of tests on the knot to realize that it can indeed slip under side load, which is why it’s considered unfit for climbing today. See Pit Schubert, Sicherheit und Risiko in Fels und Eis.

Every knot could fail if you construct a scenario that it wasn't designed for. Perhaps you could rephrase the question as "is the bowline appropriate for 'sideways' loading of the main loop?". My concern is that you asked a very broad question about a knot, then wrote a (well structured) answer about a very specific miss-application of that knot. This is troubling to me because someone new to knots might draw the wrong conclusion (bowline is BAD) because of a miss-application (what you document is that sideways loading is BAD, which, really, applies to other knots too).
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DavidRMar 12 '13 at 14:28

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@DavidR, you’re right, but the problem is that the bowline is often presented as a knot that can’t slip no matter what. And unstated non-obvious assumptions (such as that you don’t load the knot sideways) are very bad when talking about safety. Even in my answer I say that the first option is perfectly acceptable and further discussion here should make the case completely clear. Main thing is that people know the dangerous case.
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zoulMar 12 '13 at 15:01

Agreed. Could you edit the title to address this? 'Sideways' loading is actually a general issue with a certain class of knots, including both the bowline and figure-8 (i.e., either of the 2 popular knots a climber might use to tie in with will fail catastrophically when sidways loaded). I feel like this answer would be more appropriate to a differently worded question, so it would help people think about the issue clearly. If you did that, I would delete my current answer, and write up a different one about identical issues with the figure-8. That could actually be pretty helpful.
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DavidRMar 12 '13 at 15:07

I have edited the question to sound less alarmist. I didn’t want to go to the completely general case of loading different types of knots, but I think you can still slip the information about the figure-8 into your answer quite nicely.
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zoulMar 12 '13 at 16:21

In the context of rock climbing, compared to a figure-8 knot, bowlines are:

About as strong under ideal circumstances

BUT:

Can come untied on their own when unloaded

Are more difficult to visually inspect (important, because climbers frequently rely on partners to check their knots, and may be tying and untying knots when they're tired and / or distracted).

Both the figure-8 and a bowline (and its variations) will fail if loaded in the manner your picture indicates. Any knot is only useful for the scenarios it was designed for, and part of knot-craft is knowing what the knots are designed to do, not just how to tie them.

I'm going to try and find recommendations from link-able authorities, but its a little complicated because most "authorities" are either books I don't have with me, are groups like the AMGA, which don't put a lot of information free online.
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DavidRMar 12 '13 at 14:42

+1 - The bowline is great for a lot of things, just not holding a person's life.
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manofthesonMar 12 '13 at 21:40

Although the double bowline (aka bowline on a bight...depends how you tie it) is great with a yosemite backup or double overhand backup, but still harder to visually inspect and much less common than the eight.
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manofthesonMar 13 '13 at 6:04

absolutly, agreed... I wasn't sure how to addres the "yosimitee bowline", there are a couple variations I've seen (they all double the initial loop and all tie the end off with a half hitch, but sometimes the end-strand passes back through the knot, so that its on the climbing end, sometimes it ends in the main loop of the bowline), I wasn't sure if / how to address that, because I was hesitant to recommend one or the other.
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DavidRMar 13 '13 at 13:29

None of the photos show the bowline with the loose end tied off. In this (untied off) form the knot is unsafe as there is a strong chance of slippage. It's also easy to tie badly with fatal consequences. On the plus side its possible to tie one single handed in about 4 seconds.

When tied off with a single or better double hitch though then this knot is pretty safe, for most situations it still would not be my knot of choice though.

The Bowlines best application is possibly tree or post belays, but even here after moderate loading it can be hard to untie and a better choice would be round turn + 2 half hitches.

For heavy loading situations the double figure of eight (or fig 9 for super heavy) is a better knot but lacks the ease of set up.